This invention was made under, or during the course of contracts with the United States Department of Energy and the New York State Enegy Research Development Authority.
Means for concentrating solar energy are being widely studied and developed. Most systems involve the use of reflectors in various configurations. These reflectors are of two types, glass mirrors and reflecting films. When specular precision is required, glass mirrors have been the only choice because techniques for film bonding have failed to meet specular requirements; that is, when the film is bonded to a rigid substrate the bonding agent, while curing, effects a wrinkling of the reflecting film. Resultingly, the advantageous if light weight, easily supported solar energy concentrators has not been available for these systems.
Furthermore, while there are many bonding techniques for forming composite layers and which utilize non-bonding materials to prevent sticking of the product to the mold or press plates, etc. (see for instance the patent to Dickey, U.S. Pat. No. 1,352,436) and to prevent sticking of the individual layers in the composite body (see the patents to Troetzmueller, U.S. Pat. No. 2,558,632 and to Kelly et al, U.S. Pat. No. 3,969,174, which also discloses a method for producing a plastic laminate having improved surface smoothness), the art has not taught any successful technique for retaining the reflecting film in a flat condition against the action of the bonding agent between the film and its carrier or rigid substrate while the bonding agent is curing; i.e., drying.
Techniques for forming "glassless" mirrors are disclosed in U.S. Pat. No. 3,877,139 and in the patent to Benzies U.S. Pat. No. 3,434,181, but forming such mirrors with the specularity achieved by the present invention is not possible with these techniques which, to the inventor's knowledge, have not achieved specularity better than 3 milliradians.
It is therefore an object of the invention to provide a reflector of the reflecting film type and which exhibits a specularity of a very high order, for instance, of one milliradian or better and comprising a reflecting film bonded to a rigid but light weight backing or substrate.
Desirably, the substrate presents a very smooth surface for bonding the reflecting film thereto and in this resoect, when the main body portion of the substrate conveniently is made of light weight polystyrene foam or similar material, a sheet having such a smooth surface may be provided between the film and main body portion. A structural layer of a suitable sheet material may also be secured to the back of the main portion.
The method of the invention comprises the steps of initially laying the film, reflective side down, upon a smooth surface, such as glass, with a non-bonding liquid having high molecular adhesion between the film and the glass and then bonding the non-reflective side to an appropriate rigid backing. The non-bonding liquid preferably has temporary adhesion greater than the forces encountered during the application and curing of the bonding agent and handling of the assembly during production and can consist of a soap and water mixture or any such liquid which can be readily parted or otherwise eliminated after bonding.
The method of the invention lends itself to the production of relatively large reflectors of the reflecting film type.